Fuel oil furnace shudder.

My old "Sears 10" furnace has a strange shudder sound when the blower is running and it gets louder when the blower turns off.

Opening the "type R-C" draft control makes the sound louder after a 1 second delay.

The shuddering when the blower turns off "stops abruptly" before the blades stop completely.

I can eliminate the shuddering by increasing the combustion air flow by adjusting the air intake slider, but then it does not reliably light when cold.
Note: The electric eye trips the safety.

As far as I can tell I have the electrodes and nozzle adjusted to the correct placement based on a table I found on the Internet.

Q1: Could a dirty nozzle cause this?
I've taken it all apart and cleaned the tiny groves that make the fuel swirl. Then again I could have damaged it in the process...

Q2: Could this be a matter of dirty air passages?
There is some minor buildup on the fan blades and on the air swirl fins, but they aren't clogged.

---
I am also trying to reduce the BTU's produced since I had to reduce the blower cutout temperature in order to protect that cheap insulated plastic ductwork I've had to switch to due to building structural reasons.

Q3: Can I put in a nozzle with a lower flow rate?
Right now I have a .75 GPH, 80 Deg, H (Hollow pattern)

--
PS: Does anyone have a guess as to how old of a unit this thing is?
I know it does not have that many hours of run time due to being part of a "duel fuel" system. The heat exchanger for example is very clean.

It's a model number 867, but Sears has a policy of never printing dates on products to prevent people complaining about getting last years model.

I need to get this thing working perfectly or I have to replace it before I sell the place in a few years.